The present invention relates to a gas venting arrangement in a high speed injection molding apparatus, and to a method for venting gas in the high speed injection molding apparatus. More particularly, the invention relates to a mechanism for driving a gas vent control valve and a method for driving the gas vent control valve for closing the valve at high speed and at a proper timing without any delay.
In an injection molding method such as a die-casting method, a molded product often contains voids in its interior due to injection of a molten metal into a mold cavity in which gases are extant. The gases are mingled with the molten metal and remain intact, so that resultant molded product does not have high quality.
In order to remove gas from the molded product, a gas vent passage is generally provided which is connected to the mold cavity so as to discharge gas in the cavity during injection molding. More specifically, a gas vent control valve is provided at the gas vent passage. The gas vent control valve is opened during injection molding so as to allow the gas to be discharged therethrough, and is closed so as to avoid leakage of the molten metal through the gas vent control valve.
In order to maximumly discharge the gas so as to provide a void-free molded product, the gas vent control valve should have to be opened as long as possible, yet the gas vent control valve should have to be closed before the molten metal reaches the valve so as to prevent the molten metal from passing therethrough.
More specifically, generally, a vacuum sucking system is disposed at downstream side of the gas vent control valve so as to positively suck gas within the mold cavity. In order to avoid leakage of the molten metal into the vacuum sucking system, the gas vent control valve must be closed before the molten metal splash reaches the valve. The molten metal splash may be generated because of the high speed injection, application to vacuum in the mold cavity and relatively small cross-sectional area of a gate portion of the mold cavity. On the other hand, if the gas vent control valve is closed at relatively early timing, sufficient gas venting cannot be performed, so that the final molded product may contain voids, to thus lower the quality. Therefore, the gas vent control valve must be closed at an optimum timing in order to maximumly discharge the gas within the mold cavity toward outside of the molding machine, yet to avoid leakage of the molten metal through the gas vent control valve before the molten metal splash reaches the valve, that is, the valve must be closed immediately before the molten metal splash passing through the gas vent passage reaches the valve.
According to a conventional gas venting arrangement, the molten metal within the mold cavity is detected, and the gas vent control valve is closed by pneumatic pressure in response to the detection signal. One example of such conventional arrangement is disclosed in Japanese Utility Model Application Kokai No. 61-195853.
FIGS. 1 thru 3 show the gas venting arrangement disclosed in this publication. A metal mold 1 includes a stationary mold half 2 and a movable mold half 3. Parting faces 4 of the mold halves 2 and 3 are formed with a mold cavity 5 and a gas vent passage 6 in fluid communication with the cavity 5. The gas vent passage 6 has relatively large inner diameter. A gate 7 is provided at upstream side of the mold cavity 5, and the gas vent passage 6 is formed at downstream side thereof. Distal end of the gas vent passage is open to the atmosphere. Alternatively, the distal end is connected to a vacuum sucking device 8 as shown for positively discharging gas in the mold cavity 5 toward outside of the metal mold 1. The vacuum sucking device 8 includes an electromagnetic change-over valve 8a, a tank 8b, a vacuum pump 8c and a motor 8d.
At the downstream end portion of the gas vent passage 6, there is provided a tapered gas vent control valve 9 for selectively opening the gas vent passage 6 to thus allow gas to discharge therefrom. Further, a detection member 10' is disposed at the gas vent passage 6 and at the upstream side of the control valve 9. The detection member 10' detects the molten material such as electrically conductive molten metal. When the molten material is brought into contact with the detection member 10', the detection member 10' detects the molten material and sends detection signal to an electric control means (not shown), and the electric control means sends instruction signal to a valve driving mechanism. The gas vent control valve 9 is moved in response to the operation of the valve driving mechanism.
The valve driving mechanism 12 shown in FIG. 1 includes a valve driving cylinder 12d, a piston 12f integrally connected to a valve head 9a of the gas vent control valve 9 and slidable in the valve driving cylinder 12d, an electromagnetic change-over valve 12a and a compressor 12C. The piston 12f divides the driving cylinder 12d into a front chamber 12g and a rear chamber 12i. The change over valve 12a provides first and second positions. In the first position, pneumatic pressure is positively applied to the front chamber 12g by the pneumatic drive means 12C to move the piston 12f toward the rear chamber 12i, so that the valve 9 closes a tapered valve seat 12j. In the second position of the change-over valve 12a (FIG. 1 shows the second position of the change-over valve 12a), pneumatic pressure is positively applied to the rear chamber 12i to urge the piston 12f toward the front chamber 12g, so that the valve head 9a is moved away from the valve seat 12j, to thereby allow gas to pass therethrough.
In this gas venting arrangement, if molten material were to reach the gas vent control valve 9 and be discharged therefrom prior to complete closing of the gas vent control valve 9 in response to the detection of the molten material by the detection member 10', it would be impossible to conduct subsequent injection molding operation. Therefore, it is necessary to retard the molten material in reaching the gas vent control valve 9 so that the vent control valve 9 is closed prior to the molten material reaching the valve 9. Therefore, after detection of the molten material by the detection member 10', sufficient time must be provided by delaying the molten material in reaching the valve 9. For this, in the above described arrangement, the gas vent passage 6 is in the form of net pattern 6a having a plurality of obstructing protrusions 6b as shown in FIG. 2. Alternatively, the gas vent passage 6 is in the form of meandering pattern 6c as shown in FIG. 3.
Turning back to FIG. 1, a casting sleeve 14 formed with a casting port 14a is fixed to the stationary mold half 2. The casting sleeve 14 communicates with a melt runner 13 separated from the mold cavity 5 by the gate 7. An injection cylinder 15 is provided with an injection plunger 16 extending from and retracting into the cylinder 15. The plunger 16 is integrally provided with a striker 17 abuttable against a limit switch 18 and a high-speed limit switch 19 during extension strokes of the plunger 16. The limit switch 18 is electrically connected to the electromagnetic change-over valve 8a, and the limit switch 19 is electrically connected, through an injection molding drive unit (not shown), to the injection cylinder 15. The molten material supplied into the casting sleeve 14 through the casting port 14a is introduced into the mold cavity 5 through the runner 13 and the gate 7 by the extension of the plunger 16. After the plunger 16 extends to close the casting port 14a, the striker 17 abuts against the limit switch 18, so that the electromagnetic change-over valve 8a is operated. As a result, gas in the mold cavity and the casting sleeve 14 is aspirated by the pump 8c and is discharged therefrom through the valve 9.
When the striker 17 abuts against the limit switch 19, the limit switch 19 generates an instruction signal to a driver unit (not shown) to operate the plunger 16 at high extension speed, so that high speed casting is attainable.
In operation, while the valve head 9a of the gas vent control valve 9 is spaced away from the valve seat 12j, the molten material is poured into the casting sleeve 14 through the casting port 14a and the casting cylinder 15 moves the plunger 16 toward the sleeve 14 and the plunger 16 closes the casting hole 14a. Thereafter, electromagnetic change-over valve 8a is operated upon abutment of the striker 17 to the limit switch 18. As a result, vacuum pump 8c is connected to the distal end of the gas vent passage 6 for discharging gas in the cavity 5 and the sleeve 14 from the metal mold 1. In this sequence, opening of the valve 9 is maintained.
When the plunger 16 further extends to completely fill the molten material into the mold cavity 5, the molten material may flow into the gas vent passage 6 and into contact with the detection member 10'. Upon contact, closed electrical circuit is provided, since the molten material is an electrically conductive material, and the member 10' issues a detection signal. Thus, the electromagnetic change-over valve 12a is operated or is moved to a first position by the detection signal. By the change-over operation of the valve 12a, the front chamber 12g of the valve driving cylinder 12d is connected to the compressor 12C, so that pneumatic pressure is applied to the front chamber 12g. As a result, the piston 12f is urged toward the rear chamber 12i, and the valve head 9a is seated onto the valve seat 12j for closing the valve 9. Therefore, leakage of the molten material from the metal mold 1 can be prevented. In this case, since the tapered valve 9 is seated on the tapered valve seat 12j, close contact therebetween is attainable to thus further ensure prevention of melted material from leakage. After the injection molding, the movable mold half 3 is separated from the stationary mold half 2, for removing the molded product. In this product removal, flushes can be also removed from the gas vent passage together with the casted product. Upon flash removal, the electrical control means is operated to operate the electro-magnetic change-over valve 12a into the second position shown in FIG. 1. As a result, pneumatic pressure is applied to the rear chamber 12i to move the piston 12f toward the front chamber 12g, to thereby move the valve head 9a away from the valve seat 12j. This is the reset position of the gas vent control valve 9.
Another conventional gas venting arrangement is disclosed in Japanese Patent Application Kokai No. 60-49852. In this arrangement, a gas vent valve is closable by the inertial force of molten material if the inertial force of the molten metal is sufficiently large, or by an actuator which responds to a signal from a temperature sensor which detects the metal mold temperature if the inertial force of the molten metal is small. When the metal mold temperature detected by the detector is lower than a preset temperature, an electrical signal is sent to the actuator. In operation, during ordinary metal injection, the metal mold temperature is higher than the preset value, such that an electromagnetic valve is not operated and the gas vent valve is closed by the inertia of the molten material. During an initial start-up period of or in special occasions where the molten metal temperature is lower than the preset value and accordingly, the molten metal does not provide large inertial force, the electromagnetic valve is actuated, resulting in closure of the valve. The sensor does not always control gas vent valve opening and closing.
Still another conventional gas venting arrangement is disclosed in East German Patent No. 146,152 which is directed to a seal for vacuum pressure die casting dies in which molten metal enters a riser. A contactor positioned within the riser is contacted by liquid metal rising in the die to close an electric circuit in which a relay is disposed for actuating a control magnet.
Still another conventional gas venting arrangement is disclosed in Japanese Patent Application Kokai No. 63-60059 in which a switching circuit is provided between a molten metal detection sensor and a drive means which drives a gas vent control valve, the drive means being one of an electromagentic valve and an electromagnetic coil.
According to the above described prior art, it would be almost impossible to promptly close the gas vent control valve instantaneously upon detection of the molten metal by the detection member. The reason therefor is summarized as follows;
(1) According to the conventional arrangement, it would be impossible to promptly generate detection signal indicative of the contact of the first molten metal splash with the detection member so as to promptly generate output signal for driving the valve driving mechanism. That is, when the molten metal is splashed, it intermittently contacts the detection member at high frequency. The molten metal is electrically conductive, so that splashed molten metal provides a pulsating voltage line or high frequency pulse as shown in FIG. 10 Section (I) at each detection of the molten metal. Here, it is quite important that the gas vent control valve must be immediately closed upon the first detection of the initial pulse (the first splashed molten metal). Otherwise the first splashed molten metal may pass through the gas vent control valve, which is extremely disadvantageous. In this regard, as soon as the detection member detects the first splashed molten metal, it is necessary to generate output signal in response to the molten metal detection signal for driving the valve driving mechanism in order to close the gas vent control valve. However, in the conventional arrangement, there were time lag for generating the output signal.
(2) According to the conventional arrangement, when the detection member detects the molten metal and send the detection signal to the electric circuit, the electric circuit generates an output signal for change-over operation of the electromagnetic valve, so that compressed air from the compressor is supplied to the valve driving cylinder. Therefore, the piston of the cylinder is displaced, so that the gas vent control valve connected to the piston rod is closed.
With the structure, operation of the valve driving mechanism requires a given time period after receiving the output signal, since the change over operation of the electromagnetic valve requires a predetermined time period. As a result, closing timing of the gas vent control valve may be retarded.
In another aspect of this type of technology, there has been drawbacks in the detection member itself. More specifically, in a conventional detection member (molten metal sensor) as shown in FIG. 4, a non-electrically conductive holder 10'e is fixedly supported to the metal mold 2, and, two electrically conductive pins 10'a and 10'b extend through the non-electrically conductive holder 10'e. End portions of these pins 10'a and 10'b are positioned at the gas vent passage 6 for detecting the molten metal. Further, an insulating member 10'c formed of a ceramic material is provided to close the holder 10'e. The insulating member 10'c fluid tightly secures these pins 10a 10'b in order to prevent the molten metal from entering into the interior of the holder 10'e. At another end portions of the pins, another insulating member 10'd is provided.
Before the molten metal reaches the pins, these pins 10'a and 10'b are electrically insulated from each other. However, if the molten metal reaches these pins, these pins are electrically connected with each other, so that molten metal detection is carried out. The lines 10'f and 10'g are connected to the pins 10a and 10b, respectively, which lines are connected to electric control means for driving the valve driving mechanism.
According to the conventional detection member, when the electrically conductive pins 10'a and 10'b are contacted with the molten metal, the temperature of the pins are elevated, and the pins are thermally expanded. In this case, since the insulating member 10'c sealingly maintain the electrically conductive pins 10'a and 10'b for avoiding entry of the molten metal into the holder 10'e, the insulating members 10'c and 10'd may be broken due to difference in thermal expansion coefficients between the metallic pins 10'a 10'b and the insulating members 10'c 10'd. (The thermal expansion coefficient of the pins is higher than that of the insulating members). In order to avoid this drawback, space may be provided between the pins and the insulating members. However, then, the molten metal may be entered through the space, so that electrically insulating function is degraded or negated.